This invention relates to the purification of phosphoric acid, and, more particularly, to the removal of vanadium from phosphoric acid.
Phosphoric acid is widely used in commerce in the manufacture of various products, such as animal feed, food products, and fertilizer. Phosphoric acid is prepared by mining phosphate-containing ores and then producing an acid from those ores. In the "wet process", sulfuric acid is contacted to the mined ore, dissolving the phosphate values from the ore into the acid. The resulting dilute phosphoric acid is then concentrated and processed to produce the required grade and purity of phosphoric acid.
In addition to the phosphate values, the sulfuric acid also dissolves other elements and compounds from the ore into the acid. These dissolved elements and compounds may be deleterious to the purity or properties of the final phosphoric acid, and therefore must be removed during the processing for certain applications. One such impurity is vanadium. The phosphate-containing ore mined in eastern Idaho typically contains about 0.1-0.3 percent vanadium. The dilute phosphoric acid initially produced in the wet process production of phosphoric acid from such ore typically contains about 0.06-0.18 percent vanadium.
This vanadium content is too large for applications such as animal feed supplements. The vanadium content must be reduced during the processing so that the phosphorus-to-vanadium weight ratio in the acid is equal to or greater than about 700:1. This phosphorus-to-vanadium weight ratio corresponds to a maximum of about 460 parts per million of V.sub.2 O.sub.5 in phosphoric acid of 42 percent P.sub.2 O.sub.5 content.
The problem of high vanadium levels in phosphoric acid produced from Idaho phosphates (as well as those of some other regions) has been known for over 50 years. There have been several techniques developed for reducing the vanadium content. In a precipitation technique such as that described in U.S. Pat. No. 2,130,579, an oxidizing agent is added to the vanadium-containing phosphoric acid to oxidize the vanadium to the pentavalent oxidation state. Insoluble compounds containing vanadium and phosphate precipitate from such solutions. With conventional commercial processing, the V.sub.2 O.sub.5 content of the filtrate is typically 500-900 parts per million, too high for use in animal feed supplements. Extension of the precipitation time to reduce the vanadium content to acceptably low levels requires excessively long precipitation times and large acid cooling requirements, is not sufficiently reproducible, and cannot achieve sufficiently low vanadium levels for some uses of the acid.
Solvent extraction has been used to reduce the vanadium levels of phosphoric acid. Such processes are described, for example, in U.S. Pat. Nos. 3,700,415; 3,374,696; 3,437,454; 3,415,616; and 3,449,074. While operable, such solvent extraction processes have the disadvantages that the organic solvent may become entrained in the product phosphoric acid, most organic solvents are flammable, and there may be formed gummy residue phases under some circumstances that could damage rubber-lined equipment.
In an ion exchange approach, such as described in U.S. Pat. No. 2,830,874, an oxidizer is added to the vanadium-rich phosphoric acid to raise the vanadium to the pentavalent oxidation state. The pentavalent vanadium is adsorbed by an ion exchange resin, reducing the vanadium content of the product phosphoric acid. The vanadium is stripped from the loaded resin with phosphoric acid to which a reducing agent has been added. The vanadium is thereby reduced to the trivalent or tetravalent oxidation state, so that it desorbs from the ion exchange resin into the strip solution. The regenerated ion exchange resin is reused. The concentrated strip solution must be disposed of or treated in some manner. Ion exchange processes can regularly reduce the vanadium content of the acid to a sufficiently low level for use in animal feed supplements. However, ion exchange processes are expensive because of the large volume of resin required, limited resin life, and the slow ion exchange rates during the loading and stripping/regeneration steps.
There is a need for an improved approach to the production of low-vanadium-content phosphoric acid, suitable for commercial application. The present invention fulfills this need, and further provides related advantages.